The present invention relates generally to adaptive channel allocation in radiocommunication systems and more particularly to automatic control channel planning in systems which utilize adaptive channel allocation.
Various methods have been introduced to efficiently utilize the limited range of frequencies available for radio communications. One well-known example is frequency reuse, a technique whereby groups of frequencies are allocated for use in regions of limited geographic coverage known as cells. Cells containing the same groups of frequencies are geographically separated to allow callers in different cells to simultaneously use the same frequency without interfering with each other. By so doing many thousands of subscribers may be served by a system of only several hundred frequencies.
The design and operation of such a system is described in an article entitled Advanced Mobile Phone Service by Blecher, IEEE Transactions on Vehicular Technology, Vol. VT29, No. 2, May, 1980, pp. 238-244. Commonly known as the AMPS system, this system had allocated to it by the FCC a block of the UHF frequency spectrum further subdivided into pairs of narrow frequency bands called channels. At present there are 832, 30 kHz wide channels allocated to cellular mobile communications in the United States. A table of the frequencies dedicated to mobile communications in the U.S. is shown in FIG. 1. Of the 832 available channels, there are 21 control channels dedicated each to the A-carrier and the B-carrier. These 42 control channels provide system information and cannot be used for voice traffic. The remaining 790 channels, known as voice or traffic channels, carry the burden of voice or data communication.
Frequency planning is a process by which individual channels are assigned to cells within the network. Currently, most frequency planning is done a priori; that is a fixed frequency plan is "hard-wired" in place by each cellular system operator. This is known as fixed channel allocation, or FCA. However, as interference and traffic load are time varying, FCA has disadvantages with regard to system adaptability. For example, in microcells, picocells, and indoor cellular or PCS systems, the base stations are located so densely and the environment is so unpredictable and time-varying (e.g., opening a door changes the interference conditions), that channel planning becomes nearly impossible. Because of the time varying nature of the interference, therefore, an adaptive scheme can offer significant advantages.
Adaptive channel allocation, or ACA, is a method of dynamically allocating frequencies throughout a cellular system to increase system capacity and adaptability. Under an ACA scheme, more frequencies would be allocated to busy cells from more lightly loaded cells. In addition, the channels can be allocated such that all links have satisfactory quality. A common feature of ACA systems is that they allocate a channel out of a set of channels which fulfills some predetermined quality criteria. However, different ACA schemes select channels from the set based upon different criteria.
The concept of ACA is well-known to those skilled in the art, and its potential has been described in various publications. For example, "Capacity Improvement by Adaptive Channel Allocation", by Hakan Eriksson, IEEE Global Telecomm. Conf., Nov. 28-Dec. 1, 1988, pp. 1355-1359, illustrates the capacity gains associated with a cellular radio system where all of the channels are a common resource shared by all base stations. In the above-referenced report, the mobile measures the signal quality of the downlink, and channels are assigned on the basis of selecting the channel with the highest signal to interference ratio (C/I level).
Another approach is described by G. Riva, "Performance Analysis of an Improved Dynamic Channel Allocation Scheme for Cellular Mobile Radio Systems", 42nd IEEE Veh. Tech. Conf., Denver, 1992, pp. 794-797 where the channel is selected based on achieving a quality close to or slightly better than a required C/I threshold. Furuya Y. et a., "Channel Segregation, A Distributed Adaptive Channel Allocation Scheme for Mobile Communications Systems", Second Nordic Seminar on Digital Land Mobile Radio Communication, Stockholm, Oct. 14-16, 1986, pp. 311-315 describe an ACA system wherein the recent history of link quality is considered as a factor in allocation decisions. In addition several hybrid systems have been presented where ACA is applied to a small block of frequencies on top of an FCA scheme. Such an example is presented in Sallberg, K., et al., "Hybrid Channel Assignment and Reuse Partitioning in a Cellular Mobile Telephone System", Proc. IEEE VTC '87, 1987, pp. 405-411.
Apart from increases in system capacity, adaptive channel allocation can obviate the need for system planning. Planning is instead performed by the system itself. This feature of ACA is particularly attractive when system changes are implemented, when new base stations are added, or when the environment changes, for example by the construction or demolition of large buildings.
The above described adaptive channel allocation schemes, however, have generally been utilized only in conjunction with the allocation of traffic channels, and not control channels. Thus, although each base station has access to all the traffic channels, the allocation of control channels has typically remained a fixed allocation in which each base station uses a certain predetermined control channel or channels. Since the control channels are not adaptively allocated, the operator has to plan these channels geographically, i.e., which base gets what control channel so as to minimize the amount of co-channel interference experienced on the control channels. Thus, the advantages of increased capacity and adaptability realized in ACA traffic channel allocation have generally not been achieved with respect to control channel allocation. Because control channels have been fixed to each base station, changes in control channel allocation have required costly system reconfiguration. However, only if both the traffic channels and the control channels are automatically allocated is an operator effectively relieved from planning the system.
A partial solution to the problems of fixed control channel allocation could be provided by a system which directly incorporated the allocation of control channels into a conventional ACA scheme. However, allocation of traffic channels in ACA routines is based on certain criteria such as interference, channel success rate, previous performance of the channel, etc., whereas criteria for measuring quality are quite different for control channels. For example, there is no success rate of previous performance for control channels since (1) a control channel cannot be allowed to be unsuccessful, and (2) the performance of different control channels cannot be compared because this would require alternatively using each of the control channels to get an average performance measure. The latter is not desirable, since control channel allocation should remain reasonably stable.
Another problem with incorporating control channels directly into a conventional ACA routine is that transmission on control channels is bursty and irregular, particularly on the uplink from mobile to base, because the many mobile stations transmit control signals over a range of different distances and power levels. Consequently, measurements of these bursty control signals do not provide a reliable indication on which to base ACA decisions. Thus, the incorporation of control channels directly into a conventional ACA routine is not a desirable solution to the problem presented by the lack of a mechanism for adaptively allocating control channels.
There is a need in the industry, therefore, for a system and method of automatic control channel planning in ACA systems which provide reliability and system adaptability in the allocation of control channels.